A navigation system locates the position of the vehicle in which it is installed (hereinafter, “self-vehicle”) based on electric waves from GPS (Global Positioning System) satellites, and applies the travel distance and the travel direction with the use of a vehicle speed sensor and a gyro sensor, to accurately estimate the present position of the self-vehicle.
However, when electric waves cannot be received from the GPS satellites, the error in the position located by the autonomous navigation method is amplified along with the passage of time. Thus, the accuracy of the position gradually declines.
Accordingly, various methods have been proposed for correcting the position of the self-vehicle located by the autonomous navigation method. For example, map matching is for correcting the position located by the autonomous navigation method with the use of map data of the navigation system (see, for example, patent document 1). Patent document 1 proposes a method of selecting, from map data, a road whose position and orientation best match the position and orientation detected by the autonomous navigation method, and correcting the detected position and orientation by associating them with the selected road.
However, the map data included in typical commercially available navigation systems is not so accurate. Furthermore, in the map data, the road network is expressed by linear links joining intersections (nodes). Thus, the map data may not match the actual roads. Accordingly, with the map matching method, the position of the self-vehicle may not be sufficiently corrected.
Furthermore, there is proposed a navigation device for calculating the distance between the self-vehicle and an intersection when an intersection symbol such as a traffic light or a crosswalk is detected in an image photographed by a camera, and correcting the position of the self-vehicle in accordance with the calculated distance (see, for example, patent document 2). According to patent document 2, the position of the self-vehicle with respect to the traveling direction can be corrected by calculating the distance between the self-vehicle and the intersection, even while traveling on a long straight road.
Patent Document 1: Japanese Laid-Open Patent Application No. 2002-213979
Patent Document 2: Japanese Laid-Open Patent Application No. H9-243389